With the gradual exhaustion of conventional fossil energy and increasingly serious problems of global climate change and environment pollution, a clean, low carbon and sustainable green energy is to be urgently exploited, to satisfy increasing energy demand of human society and ease environmental burden. Distributed photovoltaic power generation in distributed generators is an effective way of using solar energy with a high conversion efficiency, is beneficial for making full use of resource, and improves reliability of energy supply.
Direct current distribution grids are a new development direction for distribution grids in the future, and draw wide attention in academia in recent years, and demonstration projects of the direct current distribution grids are put into application. The direct current distribution grid has advantages of flexible and open, and the inversion step for accessing a direct current generator such as a photovoltaic generator is cancelled. Therefore, the cost for accessing the distributed generator into the distribution grid is reduced greatly, and a large number of the distributed generators can be accessed.
Similar to an alternating current access, there is an anti-islanding technical problem for the distributed photovoltaic generator accessed into the direct current distribution grid. FIG. 1 is a schematic diagram showing that a distributed generator is accessed into a direct current distribution grid.
FIG. 1 shows a two-end direct current distribution grid, in which the power systems at the two ends are an alternating current system 1 and an alternating current system 2 respectively, the alternating current system 1 is connected to the direct current distribution grid through a first AC/DC interface transverter 100a, the alternating current system 2 is connected to the direct current distribution grid through a second AC/DC interface transverter 100b, a distributed generator 300 is connected to the direct current distribution grid through a grid-connected DC/DC transverter 200 and a breaker 400; and a local load 500, other load 600 and other generator 700 are also connected into the direct current distribution grid.
It should be prevented that in a case that the distribution grid stops operation, the distributed generator 300 cannot quit operation as scheduled, but continues to power the local load 500, which forms an unscheduled power island in the distribution grid stopping operation. Particularly, in a case that output power of the distributed generator 500 is basically the same as that of the local load 500, an island with relatively stable power is formed, thereby causing a non-detection zone.
The islanding results in hazards that the stability of power supply of the island and the quality of the electric energy cannot be ensured, the island with electricity affects a low-voltage recloser, and threatens personal safety and device safety in repairing, and disputes for rights and liabilities of power supply are likely caused. Different from an alternating current distribution grid, in the direct current distribution grid, abnormal frequency cannot be detected, and an active frequency shift cannot be performed to avoid the non-detection zone, hence, it is more difficult to prevent the islanding.
Therefore, it is necessary for those skilled in the art to provide an anti-islanding method for a distributed generator in a direct current distribution grid and an anti-islanding device therefor, which can accurately and timely detect whether the islanding occurs for the distributed generator, and control the distributed generator to stop operation in a case of the islanding.